1. Field of the Invention
The present invention relates to a method of forming a subbing layer on a film of or paper coated with a resin, e.g., a polyester resin, and a photographic element containing a film of or paper coated with a resin, e.g., a polyester resin having a subbing layer thereon. Particularly, the present invention relates to a method of forming a subbing layer on a linear polyester film for photographic use which is biaxially stretched, oriented and crystallized.
2. Description of the Prior Art
It is difficult to obtain a sufficient adhesive strength between a hydrophilic photographic emulsion and a film of or paper coated with a resin, e.g., a polyester resin, because resins such as polyester-type high molecular weight materials are hydrophobic since they have a high crystallinity and are linear high molecular weight materials having chemically inactive functional groups.
As a means for improving the adhesion between a film of or paper coated with a resin, e.g., a polyester resin, and a photographic emulsion in the prior art, many methods wherein the film of or paper coated with a resin is treated so as render the surface hydrophilic are known. For example, (1) a method for improving adhesion which comprises carrying out a surface treatment such as a chemical treatment (U.S. Pat. Nos. 3,145,242 and 3,376,208, etc.), a flame treatment (U.S. Pat. No. 3,072,483 and British Pat. No. 788,365, etc.), an ultraviolet light treatment, (U.S. Pat. Nos. 3,475,193 and 3,360,448, etc.), a high frequency treatment, a glow discharge treatment, an active plasma treatment or an ultraviolet laser treatment, etc., and applying a photographic emulsion to the treated surface of the film or coating, and (2) a method which comprises applying a primer to the film or coating the surface which has been subjected to the above described surface treatment (1) and applying then a photographic emulsion thereto are known.
However, these methods have many defects as set forth below. Chemical treatment has the defect that the composition of the solution changes with the lapse of time. Further, steps such as rinsing and drying in order to remove residual treating solution (e.g., sulfuric acid and chromic acid, etc.) after the chemical treatment must be conducted. Furthermore, a method which comprises carrying out a surface treatment using a solvent which swells or dissolves the resin (etching solution) is known. However, in this case, difficulties with respect to the planarity of the film of or paper coated with a resin occur because the modulus of elasticity of the resin support decreases due to the etching agent used and contraction occurs in the latter part of the drying step. In carrying out drying under conditions whereby the planarity of the support can be well maintained, fog sometimes occurs in the photographic emulsions coated thereon because of residual swelling agent or solvent in the resin support. In addition, the swelling agent is quite permeable to the skin or has an offensive smell resulting in a deterioration of the ability to handle the swelling agent. Particularly, phenolic compounds have the disadvantage of pathologically adversely affecting users who are in contact with these compounds over a long period.
In the ultraviolet light treatment, the adhesive strength to the emulsion layer barely increases even if the film of or paper coated with a resin is exposed to ultraviolet light at normal temperature. In order to obtain good adhesion in this method, the film of or paper coated with a resin support must be heated to a high temperature (for example, above 100.degree. C.) during the application of the ultraviolet light.
In this case, the support planarity is easily damaged due to a decrease in the modulus of elasticity from the ultraviolet light treatment, a contraction from the heat and the uneveness of the heat transmission due to a difference in thickness. Further, where the planarity of the support is damaged by contraction, difficulties, for example, wrinkles, occur during this step. In addition, this method is not desirable since ultraviolet lamps have a short high intensity life and are very expensive.
In the corona discharge treatment, when electrostatic charges are accumulated on the surface of the film of or paper coated with a resin by corona discharging, the charges are injected into the film to become a leakage current, and a polar group having a strong dipole moment in the resin functions as a trapping center to the movement of the accumulated charges. A lower molecular weight polymer layer which is formed by oxidation and deterioration is formed on the surface of the resin film subjected to corona discharging. Although the resin film subjected to such a treatment has good printability, the improvement of adhesion for photographic emulsions which require a high stripping resistance for the hydrophilic layer is insufficient. Further, it is necessary to heat the resin support to the temperature above a glass transition temperature thereof, and thus the same undesirable results as in the case of the ultraviolet light treatment occur.
In other methods for surface treatment, many difficulties occur and sufficient adhesion can not be obtained.
Namely, in these surface treatments, the surface of the photographic film is decomposed by oxidation. Accordingly these treatments sometimes cause a fatal defect in the photographic industry, because not only does the quality of the resin deteriorate but also fogging of the photographic emulsions easily occurs due to a very small amount of oligomer or acetaldehyde formed by the decomposition. In carrying out these surface treatments, since the limiting viscosity of the resin is reduced and the molecular weight thereof is decreased, disadvantageously the properties remarkably change when the waste polyester is reused. Further, elimination of pollution problems must be considered, because a resin, such as a polyester, is a comparatively expensive high molecular weight material and the cost of disposal is high because the polyester resin is difficult to burn or melt. In order to reuse the resin and to hold down the cost of additional equipment, a method which does not require a surface treatment would be advantageous.
A method for improving adhesion between the polyester film and the photographic emulsion layer using an emulsion polymer as a primer without applying any surface treatment has been described in, for example, U.S. Pat. No. 3,615,556 (Japanese Patent Publication No. 35641/1973), wherein a copolymer of (1) butadiene, (2) an ethylenically unsaturated carboxylic acid and (3) one other ethylenically unsaturated monomer copolymerizable therewith is superimposed on a polyester film. However, it has been found that this method has a very important defect as shown in the following examples when it is utilized as a method of forming a subbing layer for photographic emulsions. Namely, it has been found that adhesion between the photographic emulsion layer and the polyester film is quite insufficient when the polyester film is subjected to intense external stress, for example, from an automatic developing apparatus, while undergoing chemical change during development, fixing and rinsing or treatment in the presence of chemicals. This is because the copolymer used in the prior art has only acid functional groups, giving rise to the following defects: (1) self-cross-linking is insufficient when applied to the polyester film and dried, (2) the adhesive strength is remarkably reduced because the copolymer readily dissolves in conventional developers containing sodium sulfite and hydroquinone, and (3) each coated surface exhibits a blocking phenomenon to each other due to absorption of moisture in the air by the free acid groups after the film is rolled in a dry state giving rise to difficulties and resulting in a non-uniform quality with the lapse of time after winding. This blocking phenomenon is observed in Example 2 of U.S. Pat. No. 3,615,556. In that patent, an improvement has been attempted by removing the blocking which comprises increasing the styrene content, that is, decreasing the butadiene content, to reduce the residual amount of unsaturated double bonds in the copolymer or by applying an aqueous gelatin solution containing 3 wt.% of formaldehyde based on the weight of the gelatin to a surface of the butadiene copolymer layer as a double layer method where the acid group content is increased 4 or 5 times by weight.
However, according to the present invention, it is possible to form a subbing layer having a good anti-blocking property using only one application. Further, it has been found in the present invention that the subbing layer has good resistance to scratching as shown hereinafter, excellent anti-blocking properties and an improved slipping effect, which are superior to the case where a polymer having only acid groups which forms a soft subbing layer is used.
In addition, according to the above described patent, the disclosure is that a preferred degree of adhesion can not be obtained if the acid content is below 0.5% by weight (Japanese Patent Publication No. 3564/1973, column 4, line 30 although this is not described in detail in the U.S. patent), which is very different from the present invention. In using the copolymer of the present invention, there is the advantage that organic solvents are not substantially used. Furthermore, using the copolymer of the present invention, a surface treatment of the film of or paper coated with a resin is not always necessary.